The U.S. government has imposed emission standards on automobiles in an effort to control certain types of air pollutants. These emissions standards have become increasingly more stringent over the past several years, and are likely to continue to become increasingly stringent. Automobile manufacturers have employed various devices to catalytically convert certain exhaust gases into less noxious forms. Additionally, manufacturers have utilized devices to achieve more complete combustion of fuels. These latter devices have included valves which direct additional air into the engine and/or valves which recirculate some exhaust gases for further combusion.
It has been found that devices which effect more complete and efficient combustion also cause higher combustion temperatures and correspondingly hotter exhaust gases. These hotter exhaust gases have been difficult to safely accommodate. Specifically, the hot exhaust gases have substantially elevated the temperature of the exhaust pipe, the muffler, and the tailpipe. These temperatures have been high enough to damage adjacent parts of the vehicle.
Certain exhaust system accessories are known which help isolate portions of the exhaust system from adjacent parts of the vehicle. For example, heat shields which are stamp formed from sheet metal can be mounted between, but spaced from, the exhaust system component and an adjacent part of a vehicle. Heat shields typically are used to isolate a specific hot spot, such as a catalytic converter, from a particularly vulnerable part of the vehicle. Larger heat shields covering a long exhaust pipe or tailpipe have been too costly to make and too difficult to mount.
Other structures have been developed which utilize a thermal insulation. Although structures of this type are quite effecive in isolating hot spots in an exhaust sytstem, the insulation must be protected against environmental damage. This generally requires a structurally secure protective enclosure. The added costs of both the insulation and the enclosure make this otherwise effective option somewhat impractical. In practice, thermal insulation is generally used only in certain converters and mufflers. Thermal insulation has been considered too costly for most applications on elongated pipes extending throughout the exhaust system.
Air gap pipes provide an exceptional heat insulation for the exhaust pipes and tailpipes of an exhaust system. The air gap pipe includes an inner pipe through which the exhaust gases are carried and an outer pipe spaced therefrom. The air gap between the inner and outer pipes provides the desired insulation.
Until recently, the manufacture of air gap pipes has been slow and costly. More particularly, until recently, the prior art included two methods for making an air gap pipe that included discontinuities along the length. The first method involved the insertion of a smaller diameter linear pipe into a larger diameter linear pipe. A filler then was inserted between the two to centrally support the inner pipe within the outer pipe. The filler had generally comprised a metallic material having a significantly lower melting point than the metal from which the inner and outer pipes where formed. Thus, the supporting material was inserted into the space between the inner and outer pipes in a liquid form and subsequently was allowed to cool and solidify. With the inner pipe thus securely supported within the outer pipes substantially along its entire length, the combination could be bent or otherwise deformed into a configuration for use on the vehicle. This combination then would be heated sufficiently to melt the supporting filler material, and thereby enable the molten material to flow from the combined structure. An air gap thus remained between the inner and outer pipes. This procedure was extremly time consuming and costly. Furthermore, the few materials that could be used as a filler were extremely costly. Variations of this procedure involved the use of a sand or lead shot filler that could be flushed out after the pipes were bent.
The second prior art method for manufacturing an air gap pipe involved the use of a band saw to cut a previously deformed outer pipe in half along its length. The two halves then were separated and supports were welded to the inner surfaces of the outer pipe halves. A similarly deformed inner pipe would then be positioned between the outer pipe halves. Once again, this was an extremly slow and costly manufacturing process. Furthermore, it was difficult to accurately and consistently cut the outer pipe longitudinally in half.
An extremly efficient air gap pipe and a method of manufacturing the same is disclosed in U.S. Pat. No. 4,501,302 which issued to Jon W. Harwood on Feb. 26, 1985 and which is assigned to the assignee of the subject invention. The disclosure of U.S. Pat. No. 4,501,302 is incorporated herein by reference. The method disclosed in U.S. Pat. No. 4,501,302 involves the bending of inner and outer pipes into substantially identical configurations which reflect the design specifications of the vehicle. This bending may be carried out on one of the available programmable bending machines which ensure a high degree of accuracy from one pipe to the next at extremely high speeds. The bent outer pipe then is cut longitudinally. The longitudinal cut preferably is carried out by a programmed cutting apparatus such as a plasma arc cutter or a laser cutter. As explained in U.S. Pat. No. 4,501,302 the resulting method of cutting provides very fast and accurate cuts. U.S. Pat. No. 4,501,302 indicates that supports are provided between the inner and outer pipes. Preferably, these supports are formed prior to the bending and cutting of the outer pipe. The preferred supports define inwardly directed dimples formed in the outer pipe prior to the bending and cutting operations. After the outer pipe has been appropriately bent and cut and after the supports are formed, the inner pipe is placed between the outer pipe halves and the outer pipe halves are secured to one another to provide a structurally secure air gap pipe. The outer pipe halves can be secured together substantially along their entire length or at selected spaced aparat locations. The spaced apart attachments, such as weldments, can affect the rate and pattern of heat dissipation.
As noted above, the air gap pipe and method of fabrication disclosed in U.S. Pat. No. 4,501,302 is extremly desirable, efficient and effective. However, improvements to the process and to the apparatus for carrying out the process are desired to overcome certain manufacturing problems and to enable an even faster rate of production than that provided by the process disclosed in U.S. Pat. No. 4,501,302.
In view of the above, it is an object of the subject invention to provide an enhanced process for manufacturing an air gap pipe.
It is another object of the subject invention to provide a process for manufacturing an air gap pipe that substantially reduces the time required to feed pipes into the cutter.
It is an additional object of the subject invention to provide a process that enables accurate but rapid cutting of the outer pipe despite variations from one outer pipe to the next.
It is a further object of the subject invention to provide a process for facilitating the rewelding of the outer pipe halves.
Another object of the subject invention is to provide a process for manufacturing an air gap pipe that will not be significantly affected by the condensation of moisture between the inner and outer pipes during use.
An additional object of the subject invention is to provide an apparatus for accurately cutting the outer pipe longitudinally in half despite minor variations from one outer pipe to the next.
Still another object of the subject invention is to provide an apparatus for securely, accurately and rapidly positioning the outer pipe halves to one another and around the inner pipe.